Our neighborhood has walls all around it except for the 3 entrances. There are about 150 homes inside. Makes for a nice little village... Our HOA is fairly amiable and everyone gets along, so I don't see any problem getting the neighborhood organized quickly to secure the entry points.
The neat thing is, many of the adjoining little block neighborhoods are similarly structured with small, effective HOA's.
I can very easily see alliances forming in our part of town and a militia formed very quickly.
I would be most interested in securing the nearest grocery stores from looters as a first tactic.
There is also a hospital about 2-3 blocks away that would make sense to secure, if they'd let us.